FIG. 1A illustrates a prior art laser driver circuit. The laser driver circuit 100 preferably includes a differential pair of transistors 108 and 109 having gate terminals coupled to a pair of differential input signals, a drain terminal of transistor 108 coupled to a power supply, a current source (IMod)110 coupled between source terminals of the differential pair of transistors 108 and 109 and a circuit ground, and an output port coupled to the anode of a laser diode 106. The output of the laser diode 106 is coupled to the drain terminal of the transistor 109. A bias current source 107 draws a DC bias current IBias through the laser diode 106. The IBias and IMod determine the average optical output power generated by the laser diode 106, while the AC signal applied by the driver circuit 100 determines the instantaneous optical output power of the laser diode 106. The laser driver circuit 100 is placed in close proximity to the laser diode 106 to minimize transmission line side effects on the laser drive signal conveyed from the laser driver to the laser diode. The distance between the laser driver 100 and the laser diode 106 is preferably no more than one eighth of the shortest wavelength (i.e., λ=c/(2πf), where f is the highest frequency of the laser drive signal) of the laser drive signal. For high speed data transmission applications (e.g., 10 Gb/s), this distance is on the order of less than a centimeter. However, if the driver circuit 100 is placed so close to the laser diode 106, the combined thermal energy emitted by the laser diode 106 and the laser driver 100 may exceed the thermal dissipation requirements of the integrate circuit package and the laser driver system.
FIG. 1B shows the optical power of a laser drive signal as a function of the current passing through the laser diode. The optical power emitted by the laser is directly related to the current passing through the laser diode 106. Referring to FIG. 1A, assuming that the differential input signals to the laser driver circuit 100 is balanced, half of the modulation current (IMod) 110 passes through transistor 108 and half passes through transistor 109. The bias current (IBias) 107 is selected to be high enough to keep the laser diode on even when a “zero” bit is being transmitted, with all the modulation current passing through transistor 108. The current through the laser diode modulates along the linear section of the curve with a midpoint around (IBias+½IMod).
FIG. 1C illustrates a laser driver circuit 102 and transmission line configuration for driving broadband signals to a laser diode 106. This circuit configuration allows the laser driver 102 and the laser diode 106 to be placed further apart, reducing or eliminating the aforementioned thermal problem. The laser driver 102 is preferably implemented by a differential pair of transistors 108 and 109 having drain terminals coupled to a power supply through a pair of impedance-matching resistors 112 and gate terminals coupled to a pair of differential input signals (In, Inb), a current source (I′Mod) 110 for modulating current through the differential pair of transistors, and a pair of output ports coupled to the drain terminals of the differential pair of transistors 108 and 109. The pair of output ports are coupled to a pair of symmetrical signal paths connected to the P and N terminals of the laser diode 106. Each of these paths includes an AC-coupling capacitor 103, a lossy transmission line 104, and a load impedance ZLoad 105 connected in series. The first signal path also includes a bias resistor RBias 114 coupled between its lossy transmission line 104 and a power supply. The second signal path also includes a bias resistor RBias 114 coupled between its lossy transmission line 104 and a bias current source (I′Bias) 107. The laser diode 106 is biased by the bias current source 107.
One of the problems of the laser driver system illustrated in FIG. 1C is that as the frequency of the laser drive signal transmitted from the laser driver 102 to the laser diode 106 increases, the laser drive signal suffers higher signal attenuation in the lossy transmission lines 104. As a result the quality of the optical signal produced by the laser diode 106 decreases as the frequency of the input signal increases.
Another problem with the laser driver circuit of FIG. 1C is its high power consumption. The modulation current I′Mod 110 produces a voltage drop across the resistors 112. Because of the voltage drop across the resistors 112, a higher power supply voltage is required to drive the laser driver integrated circuit, which in turn translates into higher power consumption for the laser driver integrated circuit. Similarly, there is a voltage drop across the bias resistor 114 generated by the bias current I′Bias. Because of the voltage drop across the resistor 114, a higher voltage power supply is required to drive the laser diode, which in turn translates into higher power consumption for the laser driver circuit.
In view of the shortcomings of the prior art, it is an objective of the present invention to provide a laser driver circuit that can compensate for the transmission line loss of a laser drive signal at a set of predetermined operating frequencies. Another objective is to provide a laser driver circuit with a low power consumption.